concrete weight

[KEmerson]

Hello,
is there a rule of thumb or real numbers for how much Portland with Vermiculite & Perlite mixed in? I know from reading here that some people use one or the other or both and to a slightly varying amount per. I know much depends on this or that... I just need a ballpark estimate. How heavy my Portland/Verm./Perl hearth is is important information for my project. What's a cu. foot going to weigh?
Thanks
Kim

[dmun]

Re: concrete weight

I think, anyone correct me if I'm wrong, but the weight of the vermiculite concrete is going to be the weight of the dry ingredients. Portland cement weighs 94 pounds per cubic foot. I can't put my finger on the weight of vermiculite: it's going to be about a quarter of the shipping weight of a four cubic foot bag.

And no, I've never heard of anyone mixing vermiculite and perlite for insulating concrete for an oven project.

[KEmerson]

Re: concrete weight

So the water weight doesn't factor in at all? I suppose that makes sense. I hadn't thought of that. Thanks. (Never too old to learn!)

But apparently I'll need to do some more reading as I thought I understood that some people mix Perlite & Vermivculite with the Portland. Is it an either/or and what's the difference? Why one over the other? I'm at the gathering information stage and a lot is being read on the fly. Details like mixing or not mixing all three has so far not been the focus. But how much this thing is going to weigh is a focus for the moment.
Thanks
Kim

[Archena]

Re: concrete weight

Um, how fast does that stuff cure? Water weight should be an issue during curing because the water is still present. Concrete isn't known for fast drying - does the perlite alter it some way?



=Disclaimer= Utter non-expert here.

[gjbingham]

Re: concrete weight

I believe the difference between the perlite and vermiculite are negligable. I think dmun's correct. The four cubic foot bag of vermiculite weighs only around ten - fifteen pounds.

Just curious why weight is such a worry. You've got a lot more weight coming before you're done.

[Breven]

Re: concrete weight

They don't look the same, but I understand the charachteristics to be the same. When I poured my insulation hearth, Home Depot only had a couple of bags of vermiculite left- so mixed in Perlite with it. Vermiculite is a tan color and almost looks like wheat or some sort of grain. Perlite is white and looks like tiny styrofoam balls.

[KEmerson]

Re: concrete weight

Weight is a concern because I want to build into one of those half school buses.

Read here hearth2hearth.com/blog

[Jed]

Re: concrete weight

Kimemmerson,

After a short career as a long haul truck driver, I have developed an eye for safety on the highways, and your concerns about a converted school bus having the capacity to carry the weight of a WFO is well founded.

My first concern would be the capacity of the vehicle, and the weight of not just the oven, but everything else that goes with the project. To get any kind of a kitchen certified by the authorities to provide food to the public, you will have to have a way to wash: hands and utensils at a minimum. Add water weight (for washing) to the wood and the oven, along with any counters, refrigerators, and miscellaneous stuff (like food), and it doesn't take much to overload a vehicle, and that puts everyone at risk... For me, the biggest concern is the ability to stop. Go a slow as you want, but when it is time to stop, it is time to stop, and if the vehicle can't stop, that is a design (or maintenance) issue that can and should be avoided.

With this in mind, I would advise you don't use a perlcrete, or vermicrete in your design. Spend the money and get the good stuff; the lightweight board insulation for under the hearth and the bat insulation for around the dome. This will save a bunch of weight. Consider using a refractory kit oven. I haven't done the math, but these will probably weigh less than a fire brick design, and they may tolerate the rigors of travel better than fire brick.

Consider building the oven on a trailer. There are several examples on this web site, and in Seattle there is a company that has a trailer unit they haul around to cater at events. This takes the weight off the bus, and onto another vehicle. You can equip the trailer with brakes to help with the work of stopping the parade. As you know, working with wood to fire an oven is messy in the sense that wood is always sloughing 'dirt', or wood chips, or bark or whatever. The point is that it is messy. Having the oven on a trailer will help keep that mess out of the kitchen, so to speak...

Good luck with your project. Lots of reading and research at this step of the game will save time and trouble later!

JED

[KEmerson]

Re: concrete weight

Kimemmerson,



With this in mind, I would advise you don't use a perlcrete, or vermicrete in your design. Spend the money and get the good stuff; the lightweight board insulation for under the hearth and the bat insulation for around the dome. This will save a bunch of weight.
JED

Except for structural support, why do I need a concrete hearth? If I can insulate (can I?) well enough and still support the dome, floor etc. must I have a concrete base at all?

[dmun]

Re: concrete weight

I work with metal for a living, and sheets of metal do strange things when they are exposed to heat, particularly on one side. The term of art is "potato chipping". Metal comes out of the rolling mill hard on the outside and softer on the inside, much like bricks do. A steel base will need lots of cross members to keep it rigid, and it wouldn't hurt to send it to the heat treaters for annealing, just for stress relief.

It's also worth mentioning that brick built ovens don't deal with vibration very well. Most of the successful mobile catering ovens are modular ovens: stronger, and fewer elements to vibrate apart.

[KEmerson]

Re: concrete weight

A friend has suggested a steel plate but I didn't like that idea. And as for modular, I am beginning to suspect I may have to consider that, though well more than half the thrill here was in the building of an oven. At least I still have a back yard that doesn't seem to be going anywhere...

Back to the hearth... Can I use the board insulation under the oven as suggested by Jed? I'd need the insulated hearth regardless of modular or brick, right? So do I have to do it with Portland & Vermiculite (or Perlite)?
Thanks all

[dmun]

Re: concrete weight

You definitely need some kind of support under your oven. Neither the insulating board nor the vermiculite concrete has any strength at all.

The insulating boards and the vermiculite concrete do the same thing. The boards do the same thing in two inches that the vermiculite concrete does in four, and it's lighter as well.

[KEmerson]

Re: concrete weight

Oh I am certain I need support, but must it be 7" of concrete? My question has more to do at this point with insulation. Would the board under the oven floor be enough insulation regardless of support? So, if I had four elephants holding the oven up for me, or had it dangling by chains from a crane and had the insulating board under the oven would it still be insulated enough to retain heat for pizza? It occurs to me (a complete newbie on most of this) that if I had a frame to support a steel plate and then had sufficient insulation, the steel "potato chipping" due to heat might not be a factor and would be sufficient support. I'm just trying to lose weight (for the oven project. My own weight is another issue) wherever possible. I read that 3" of concrete and 4" of concrete w/Vermiculite is the norm. (Or the other way around). If I had enough support why not just the concrete/Vermiculite layer? Why not just the board and maybe a layer of concrete/Vermiculite? How about the bat insulation Jed suggested? Why not that as well? Why all this concrete once the support issue is dealt with? Provided it is dealt with separately from tons of concrete?
Thank you for your patience and help.
Kim

[Archena]

Re: concrete weight

=Disclaimer= I'm no expert.


Kim,

FB actually sells a metal stand. I've seen pics of an all wood stand. Concrete is not essential from a structural POV.

As long as the insulation isn't crushed by the weight that also should not be an essential function of concrete.

Since it's on a moving platform vibration is a big issue - especially for brick. That's where the rigidity of concrete and rebar may prove to be the best way to minimize the flexion of the platform (hearth) itself. I'm dubious that 7" would be necessary for that but I have no idea how much would be.

That said, it's been done before so it's certainly possible.

Invest in really good shocks. Invest more in the best brakes you can possibly buy.

[Jed]

Re: concrete weight

Kim,

You might consider building a structural steel structure for your oven (possible a bit more consistent and less weight then four elephants..). For this, I would suggest finding a civil engineer to confirm your structure is sufficient to support the weight; a civil engineer who can advise on the details of what is appropriately strong. The details are significant. The potential loads for this structure are significant, particularly as part of a moving vehicle.

Use an appropriate rigid insulation for under the oven - this will keep the heat in the oven, away from the supporting structure, and avoid compression from the weight of the masonry in the oven.

Use a cast or modular oven.

Use bat insulation over the top of the structure.

Metal venting ducts to pull the smoke from the oven and venting the smoke to the outside (you can still expect to fill the room with smoke, as my experience says that most of the smoke goes up the chimney, but not all).

And a perlcrete or vermicrete layer to finish the exterior. This will contain and protect the exterior insulation, and provide a surface to hold paint or some other light weight finish.

This combination of materials might provide the lightest combination of materials to build a retained heat oven.

With a firm plan on materials, prepare a complete weight analysis; how much do all of these materials weigh?

If the total weight of this structure exceeds the allowable design capacity of your vehicle - find another vehicle or reconsider your idea.

Keep up your planning!

JED